The present invention is generally directed to processed for encapsulated toner compositions, and more specifically the present invention is directed to processes for the preparation of encapsulated toners by the interfacial polymerization of shell-forming monomers and the subsequent free radical polymerization of core monomer, or monomers wherein surfactants or stabilizers such as polyvinyl alcohol are avoided. Thus, in one embodiment of the present invention there are provided processes for the preparation of encapsulated toners by interfacial/free radical polymerization wherein encapsulated toner particles are formed in aqueous solution of a water soluble shell monomer, which solution excludes the undesirable surfactant or stabilizer normally used for such processes. A number of advantages are associated with the processes of the present invention in embodiments thereof such as the elimination of the need for surfactant and/or stabilizer, and avoiding processes for removing surfactant and/or stabilizer by washing the product during the post-reaction operations, especially washing with water to remove the surfactant and/or stabilizer; undesirable reaction of surfactant or stabilizer such as polyvinyl alcohol and other emulsifiers, like methylcellulose with shell monomers such as isocyanates during particle formation step is avoided thereby minimizing unpredictable particle size and suspension failure; the avoidance or minimization of the toner product to environmental humidity and electroconductivity instability since no surfactant or stabilizer is used and therefore no residual surfactant or stabilizer is present on the surface of the final toner; and cost reductions as no surfactant is selected. The elimination of the aforementioned toner washing step can enable substantial cost savings when preparing encapsulated toners. The electroconductivity instability problem when a surfactant or stabilizer is selected can result in the presence of residual surfactant or stabilizer on the toner surface, even after extensive washing in some instances, which residual surfactant or stabilizer can absorb moisture from the environment, causing undesirable changes in the electroconductivity of the toners. With the processes of the present invention, the aforementioned problem and other problems are avoided, or minimized since, for example, no surfactant or stabilizer is present and the electroconductivity stability can thus be maintained for extended time periods. The process of the present invention thus enables in embodiments thereof the generation of encapsulated toners with a controlled and stable resistivity, such as, for example, from about 1.times.10.sup.3 to 1.times.10.sup.8, and preferably from about 5.times.10.sup.4 to 1.times.10.sup.7 ohm-cm, which toners are particularly useful for inductive development processes. Also, for example, the toner compositions prepared in accordance with the process of the present invention can posses apparent bulk densities of from about 0.6 to 1.0 gram/cm.sup.3.
The encapsulated toners obtained with the processes of the present invention can be selected for a number of imaging and printing systems, including xerographic and ionographic processes wherein, for example, cold pressure fixing is selected. The aforementioned toners prepared in accordance with the process of the present invention can thus be selected for permitting the development of images in reprographic imaging systems, inclusive of electrophotographic and ionographic imaging processes wherein pressure fixing, especially pressure fixing in the absence of heat, is selected. More specifically the encapsulated toner compositions obtained with the processes of the present invention can be selected for commercial ionographic printers, such as the Delphax S9000 S6000, S4500, S3000, and Xerox 4075.TM. wherein, for example, transfixing is utilized, that is fixing of the developed image is accomplished by simultaneously transferring and fixing the developed images with pressure.
Encapsulated toners and processes for the preparation thereof are known. In a number of these processes, suspension and interfacial polymerization and interfacial/free radical polymerization methods are selected. In these known processes, there is selected the use of an aqueous phase containing surfactants or stabilizers primarily to aid the formation and stabilization of the toner particles and to prevent the coalescence of these particles. One known encapsulated toner process, disclosed in U.S. Pat. No. 4,727,011 involves the emulsification of a mixture of magnetic pigment, core monomer(s) and an oil soluble shell formation monomer in an aqueous phase containing a high molecular weight surfactant, such as polyvinyl alcohol, to stabilize the high density toner particles. This is followed by addition of a water soluble shell monomer to initiate the formation of a shell by interfacial polymerization, and subsequently, the core monomer(s) can be free radical polymerized by heating. Disadvantages associated with the aforementioned processes include the undesirable reaction of the polyvinyl alcohol with, for example, isocyanates present in the oil phase as one of the shell monomers; polyvinyl alcohol depletion can result in the loss of stabilizer efficiency, thus causing, for example, unpredictable particle size control, such as formation of undesirable particles of extremely large sizes, and suspension failure; the polyvinyl alcohol must be substantially removed by washing after polymerization of the core monomer, which washing can be time consuming and uneconomical; and some residual polyvinyl alcohol usually remains on the encapsulated toner surface product even after extensive washings, and this polyvinyl alcohol can absorb moisture and water which will adversely effect the electroconductivity of the encapsulated toner, and thus adversely effect the triboelectric characteristics thereof. Also, the absorbance of moisure usually causes the toner particles to agglomerate into large aggregates of, for example, from about 50 to about 200 microns, which can cause blocking problems in the development housing, or container in the machine systems selected for development. These and other disadvantages are avoided, or minimized with the processes of the present invention.
In copending application U.S. Pat. No. 5,045,422, D/89072, the disclosure of which is totally incorporated herein by reference, there are illustrated encapsulated toners which can be prepared by the interfacial polymerization of shell-forming monomers, followed by an in situ free radical polymerization of core binder-forming monomers. Thus, in one embodiment of the patent there is described a simple and economical method for the preparation of pressure fixable encapsulated toner compositions by interfacial/free radical polymerization methods wherein there are selected as core monomer(s) an addition type monomer or monomers, and an addition polymerizable fluorocarbon compound. Other process embodiments disclosed in the patent relate to, for example, interfacial/free radical polymerization processes for obtaining encapsulated colored toner compositions. The aforementioned toners can be prepared by a interfacial/free radical polymerization process which comprises (1) mixing or blending of a core monomer or monomers, a functionalized fluorocarbon compound, free radical initiator, pigment, and a shell monomer or monomers; (2) dispersing the resulting mixture of materials by high shear blending into stabilized droplets in an aqueous medium with the assistance of suitable dispersants or emulsifying agents; (3) thereafter subjecting the aforementioned stabilized droplets of, for example, a specific droplet size and size distribution to a shell forming interfacial polycondensation; and (4) subsequently forming the core binder by heat induced free radical polymerization within the newly formed microcapsules. The shell forming interfacial polycondensation is generally accomplished at ambient temperature, however, elevated temperatures may also be employed depending on the nature and functionality of the shell monomers selected. For the core binder forming free radical polymerization, heating thereof is generally effected at a temperature of from ambient temperature to about 100.degree. C., and preferably from ambient temperature to about 85.degree. C. In addition, more than one initiator may be utilized to enhance the polymerization conversion, and to generate the desired core copolymer binder molecular weight and molecular weight distribution. Stabilizers, dispersants or emulsifying agents such as polyvinyl alcohol are utilized for the aforementioned processes. Further, in another process aspect of the patent the encapsulated toners can be prepared without organic solvents thus eliminating explosion hazards associated therewith, and, therefore, these processes do not require expensive and hazardous solvent separation and recovery steps. Moreover, with the aforementioned process of the patent there can be obtained in some instances improved throughput yield per unit volume of reactor size since, for example, the extraneous solvent component can be replaced by liquid core monomer(s) which would serve as a diluting vehicle and as a reaction medium.
There is disclosed in U.S. Pat. No. 4,307,169 microcapsular electrostatic marking particles containing a pressure fixable core, and an encapsulating substance comprised of a pressure rupturable shell, wherein the shell is formed by an interfacial polymerization. One shell prepared in accordance with the teachings of this patent is a polyamide obtained by interfacial polymerization. Furthermore, there are disclosed in U.S. Pat. No. 4,407,922 pressure sensitive toner compositions comprised of a blend of two immiscible polymers selected from the group consisting of certain polymers as a hard component, and polyoctyldecylvinylether-co-maleic anhydride as a soft component. Interfacial polymerization processes can be selected for the preparation of the toners of this patent. Also, there are disclosed in the prior art encapsulated toner compositions containing costly pigments and dyes, reference, for example, the color photocapsule toners of U.S. Pat. Nos. 4,399,209; 4,482,624; 4,483,912 and 4,397,483. All these processes are believed to utilize an aqueous phase containing surfactant or stabilizer, such as polyvinyl alcohol, for preparation of encapsulated toners.
Moreover, illustrated in U.S. Pat. No. 4,758,506, the disclosure of which is totally incorporated herein by reference, are single component cold pressure fixable toner compositions, wherein the shell selected can be prepared by an interfacial polymerization process. A similar teaching is present in abandoned application U.S. Ser. No. 718,676, the disclosure of which is totally incorporated herein by reference. In the aforementioned abandoned application, the core can be comprised of magnetite and a polyisobutylene of a specific molecular weight encapsulated in a polymeric shell material generated by an interfacial polymerization. These processes involve the use of an aqueous phase containing surfactant or stabilizer, such as polyvinyl alcohol.
There is a need for processes for the preparation of encapsulated toner compositions wherein residual surfactant or residual stabilizer on the surface thereof is avoided. Also, there is a need for economical processes for the preparation of encapsulated toner compositions wherein the use of surfactants or stabilizers is avoided, thus enabling, for example, the elimination of process steps such as washing, and disposal of surfactant waste or stabilizer waste, and prevention of process failure such as suspension failure. There is also a need for a process for the preparation of encapsulated toner compositions which have stable triboelectrical properties, and stable blocking properties. There is also a need for a process for the preparation of encapsulated toner compositions wherein the generation of undesirable large sizes, for example greater than 40 micron particles, can be avoided if desired.